A gas turbine engine includes a compressor section, a combustion section and a turbine section. Disposed within the turbine section are alternating rings of moving blades and stationary vanes. The rings, or stages, of vanes and blades are situated such that the axes of the rings are co-located with the axis of the gas turbine engine. The blades are mounted on a disk which rotates about its central axis. As hot combustion gases exit the combustor and pass through the turbine section, the disks with blades mounted thereon are rotatably driven, turning a shaft, and thereby providing shaft work for driving the compressor section and other auxiliary systems. Higher gas temperatures mean that more work can be extracted from the gases in the turbine section, thus increasing the overall efficiency of the gas turbine engine. By using cobalt and nickel-base superalloy materials, which maintain mechanical strength at high temperatures, the operating temperature capability of the turbine section is increased.
The stationary vanes disposed between the rings of moving blades stabilize and direct the gas flow from one stage of moving turbine blades to the next stage. The stabilization of the gas flow optimizes the amount of work extracted from the hot gases in the turbine section. The most efficient operation occurs when the nozzle flow areas, defined as the spaces between adjacent vanes in a vane ring, are all approximately equal.
In order to group vanes so that all nozzle flow areas in an assembled vane ring are approximately equal, the vanes are classified in groups according to ranges of flow area. For one particular engine model, the acceptable flow areas for one stage of stationary vanes range from 1.868-1.894 square inches.
In service, deterioration of the vane surface occurs due to oxidation, cracking and metal erosion caused by abrasives and corrosives in the flowing gas stream. In addition, the high gas pressures at high temperature cause distortion of the vanes, thereby enlarging the nozzle area with a consequent loss in turbine efficiency. During periodic engine overhauls, the vanes are inspected for physical damage and measured to determine the degree of flow area change and the effect on nozzle flow area classification. Before these vanes can be returned to the engine, any physical damage must be repaired and the vanes otherwise reclassified.
Several methods exist for repairing the worn or damaged vanes and for returning the nozzle gas flow area to the original classification. Repair methods include, for example, conventional fusion welding, plasma spray as described in, for example, U.S. Pat. No. 4,878,953, and the use of a tape material containing a mixture of a binder and a metal alloy powder which is compatible with the substrate alloy. The metal alloy mixture is formed into a flexible tape of uniform thickness and backed with an adhesive. After the tape is added to the vane, the vane is heated to a temperature at which the adhesive and binder decompose and at which diffusion occurs between the alloy powder and the substrate alloy. This latter technique is described in U.S. Pat. No. 4,726,101 issued to Draghi et al., which is incorporated herein by reference.
Methods for reclassifying the vanes to change the area between adjacent vanes include, for example, hot striking or otherwise bending the trailing edge of the vane, and adding material to the airfoil surface to change its contour, using, for example, the techniques listed above for vane repair. Hot striking, in addition to requiring a very expensive set of contour dies and end locators for each vane configuration, can cause cracking in the airfoil or distortion of the vane platform. The contour dies and locators experience wear and distortion during use, and require frequent refurbishment. Hot striking does not provide a consistent amount of reconfiguration due to dimensional variations in the vanes. Additionally, hot striking is not permitted on several high temperature alloys used in gas turbine engines due to the deleterious effects on material properties such as fatigue strength.
Reconfiguring by building up large amounts of material on the surface of the airfoil is very labor intensive and wasteful of the materials used for building up the surface, making the process very expensive. Consequently, a need has arisen for a cost effective method of repairing and reclassifying turbine vanes.